Dysregulation in central serotonergic systems has been implicated in the pathogenesis of major depression, a relatively common neuropsychiatric illness that is associated with considerable morbidity and mortality. We have developed and applied a new paradigm for studying serotonergic function in humans: the clomipramine (CMI) challenge test. This approach, which measures the neuroendocrine response to an intravenous dose of the serotonin (5-HT) reuptake inhibitor CMI, provides a relatively specific index of 5-HT function. We have demonstrated that depressed patients, compared to carefully matched healthy control subjects, have dysregulated 5-HT function, as manifested by blunted neuroendocrine responses to CMI challenge. This abnormality appears to be a trait marker, which persists even after clinical response to pharmacotherapy. The overall goals of our current proposal are: (1) to expand our investigations to include an important, yet relatively understudied population: adolescents with major depression; and (2) to delineate more precisely the specific 5-HT receptor subtype(s) which could account for the abnormal neuroendocrine response to CMI challenge in depressed patients. We plan to test several specific hypotheses: 1) Major depression in adolescent patients is associated with dysregulation in central serotonergic systems. We will test this hypothesis by examining the prolactin and cortisol responses to intravenous CMI challenge in 41 adolescent patients with major depression and 41 matched healthy controls. 2) The pathophysiologic mechanisms responsible for the blunted neuroendocrine responses to CMI in depressed patients involves 5-HT1A and 5-HT2 receptors, but not 5-HT3 receptors. This hypothesis will be tested by comparing the prolactin and growth hormone response to CMI challenge in healthy volunteers who are exposed to either a specific 5-HT1A, 5-HT2, or 5-HT3 receptor antagonist, or placebo. We predict that pre-treatment with either a 5-HT1A antagonist or a 5-HT2 antagonist will lead to a response profile to CMI challenge that mimics the response seen in depressed patients. We also propose a series of parallel experiments in animals, which will provide greater access to specific methods and pharmacological interventions that are not available for use in humans. These preclinical experiments will utilize the Fawn- Hooded (FH) rat, a strain with central and peripheral 5-HT dysfunction and exaggerated immobility in the forced swim test, an animal model for depression. We plan to test the following hypothesis in these experiments: 1) The FH rat will demonstrate a different pattern of neuroendocrine response to specific 5-HT receptor subtype agonists, compared to control rats. These differences will become manifest in early puberty. 2) Following chronic antidepressant treatment, there will be a reduction in the exaggerated immobility in the FH rat, accompanied by further reduction in the neuroendocrine response to 5-HT1A and 5-HT1B agonists. Experimental proof of the hypotheses to be tested will expand our understanding of the pathogenesis of depression to include an important population, i.e., adolescents, and should further define the precise pathophysiologic mechanisms of this illness.